mtx: reduce lock accesses

Instead of spuriously re-reading the lock value, read it once.

This change also has a side effect of fixing a performance bug:
on failed _mtx_obtain_lock, it was possible that re-read would find
the lock is unowned, but in this case the primitive would make a trip
through turnstile code.

This is diff reduction to a variant which uses atomic_fcmpset.

Discussed with:	jhb (previous version)
Tested by:	pho (previous version)

0 files changed, 0 insertions, 0 deletions